Video data is typically provided in one of two formats, composite video or S-video. Composite video is a video signal in which the luminance (brightness), chrominance (color), vertical blanking information, horizontal sync information, and color burst information are presented on a single channel according to a particular coding standard. S-video, or separated video, is a higher quality video delivery system in which the luminance, vertical blanking information, and horizontal sync information are carried on one channel and the chrominance and color burst information are carried on another channel. The different coding standards for composite video and S-video include the National Televison Standards Committee (NTSC) standards, the Phase Alternate Line (PAL) system, and the Sequential Couleur A'manorie (SECAM) standard.
Under NTSC standards, one frame of video data contains two fields of video data. One frame of video data has 525 horizontal lines of information with each of the two fields having 262.5 horizontal lines of information. The horizontal lines of information are scanned onto a monitor, such as a television set, at a rate of 30 frames per second.
The prior art of video capture and preview requires video data to go through multiple processing steps during capture and preview. The first step in video capture and preview is the conversion of an analog video signal into raw digitized video samples and then the conversion of raw digitized video samples into digital image data. Raw digitized video samples consist of digitized values of the analog signal. Digital image data consists of data that has been decoded into color pixel values.
After the analog video signal has been converted into digital image data, the digital image data is either immediately transferred for preview or stored in memory until it is called on for previewing. Digital image data that is selected for previewing is reconverted from digital image data to analog video data and then displayed on a monitor. Existing video capture and display devices accomplish the conversion and reconversion of analog video data and digital image data with the aid of general purpose video decoding and encoding chip-sets. The chip-sets perform the necessary timing recovery, luminance/chrominance separation and chrominance demodulation tasks in real-time.
Video capture and preview techniques are often applied to video printing. Video printing is a technique whereby a still image is printed from motion video such as a VHS tape. In video printing, a video is viewed by a user. Once an image of interest is identified, a still video image is created for previewing on a monitor. Creating the still video image involves converting corresponding digital image data into an analog video signal. The user then previews the still video image and directs the image to be printed if the image is desired. Once selected, the digital image data that corresponds to the still video image is transferred to a printer for printing.
Prior art in video printing is disclosed in U.S. Pat. No. 5,045,951 to Kimura et al. and U.S. Pat. No. 4,998,215 to Black et al. In Kimura et al. a video signal processor and a video signal processing method for a video printer are disclosed. Fundamental to the invention is an initial analog-to-digital conversion of the original analog video signal. The digital data output from the analog-to-digital convertor, according to Kimura et al., is pixel data. As stated above, pixel data is data that is in a format that computers recognize as image data. That is, no additional conversion of the pixel data is necessary before printing. But, in order to preview the data that has been selected for printing, the pixel data must be reconverted into an analog signal that can be displayed on a monitor. The reconversion requires additional hardware and processing.
In Black et al. a method and apparatus for converting video information for printing by a standard printer are disclosed. The first process in the invention is an analog-to-digital conversion. The analog-to-digital convertor converts an analog video signal into display dots, or pixel data. The pixel data is in a standard graphics format that is compatible with a standard printer. But, if a user wants to preview the still image before it is printed, the pixel data must first be reconverted into an analog video signal that is compatible with a video monitor.
While the prior art techniques for video capture and preview work well for their intended purpose, there are limitations. For example, the conversions from analog video data to digital image data and back to analog video data for previewing can cause loss of picture quality. Also, 100% of the digital image data must be stored in memory for potential previewing or printing. The storage requirements can quickly overload storage capacity. In addition, the chip-sets required to perform analog-to-digital and digital-to-analog conversions have a relatively high cost.
What is needed is a method and system for previewing a still video image without first having to convert an analog video signal into digital image data and then having to reconvert the digital image data back into an analog video signal representative of the desired still video image. In addition, what is needed is a way to store video signals so that memory space is effectively utilized while data integrity is kept intact.